Abstract:

A fiber laser amplifier system including a beam splitter that splits a
feedback beam into a plurality of fiber beams where a separate fiber beam
is sent to a fiber amplifier for amplifying the fiber beam. A tapered
fiber bundle couples all of the output ends of all of the fiber
amplifiers into a combined fiber providing a combined output beam. An end
cap is optically coupled to an output end of the tapered fiber bundle to
expand the output beam. A beam sampler samples a portion of the output
beam from the end cap and provides a sample beam. A single mode fiber
receives the sample beam from the beam sampler and provides the feedback
beam.

Claims:

1. A fiber amplifier system comprising:a splitter splitting a feedback
beam into a plurality of fiber beams;a plurality of fiber amplifiers each
receiving and amplifying a fiber beam, said fiber amplifiers each
including an output end;a tapered fiber bundle including an input end and
an output end, said input end being coupled to the output end of all of
the fiber amplifiers, said output end of the tapered fiber bundle being a
combined fiber including a portion of all of the fiber amplifiers with
fiber cores in a single fiber mass, said tapered fiber bundle outputting
a combined output beam;an end cap optically coupled to the output end of
the tapered fiber bundle, said end cap expanding the output beam from the
tapered fiber bundle;a beam sampler for sampling a portion of the output
beam from the end cap and providing a focused sample beam; anda single
mode fiber receiving the focused sample beam from the beam sampler, said
single mode fiber providing the feedback beam.

2. The system according to claim 1 further comprising a polarization
detector, a synchronous polarization processor and a plurality of
polarization controllers each receiving one of the fiber beams, said
polarization detector detecting the polarization of the fiber beams in
the sample beam and providing polarization measurement error signals to
the polarization processor, said polarization processor controlling the
polarization controllers to control the polarization of the fiber beams
in the fiber amplifiers in response to the polarization measurement error
signals to make the orientation of the polarization of the fiber beams
same.

3. The system according to claim 2 wherein the polarization detector
detects the polarization of the fiber beams by detecting a unique dither
on the fiber beams in phase or amplitude using distinct frequencies for
frequency modulation or amplitude modulation or a distinct code for code
division multiple access or time division multiple access.

4. The system according to claim 1 further comprising a collimating and
magnifying telescope receiving the output beam from the end cap before
the beam sampler, said collimating and magnifying telescope expanding and
collimating the output beam.

5. The system according to claim 1 wherein the beam sampler is part of a
combined lens and sampling grating assembly including a lens and a
sampling grating, said lens collimating the output beam from the end cap.

6. The system according to claim 1 further comprising a pre-amplifier,
said pre-amplifier receiving the feedback beam in the single mode fiber
and amplifying the feedback beam before it is sent to the splitter.

7. The system according to claim 1 wherein the tapered fiber bundle
includes a plurality of coreless cladding fibers positioned around the
fiber amplifiers.

8. The system according to claim 1 wherein the tapered fiber bundle
includes a low index of refraction glass tube provided around the fiber
amplifiers.

9. The system according to claim 1 wherein the fiber amplifiers are
coupled together into a multi-core fiber where each fiber in the
multi-core fiber includes an air cladding layer, said multi-core fiber
being chemically etched at one end to separate constituent fibers, and
wherein the individual fiber amplifiers are coupled to the multi-core
fiber by splices to said constituent fibers, and the other end of said
multi-core fiber is tapered to form to the tapered fiber bundle.

10. The system according to claim 1 wherein the end cap includes a
negative gradient index lens and a uniform glass rod optically coupled
together where the negative gradient index lens is coupled to the output
end of the tapered fiber bundle.

11. The system according to claim 1 wherein the end cap includes an
anti-reflective coating provided on an output end of the end cap.

12. The system according to claim 1 wherein the end cap includes a
positive gradient index lens coupled to an output end of the end cap or a
positive lens formed by a curved surface at an output end of the end cap.

13. The system according to claim 12 wherein the positive lens is part of
a collimating and magnifying telescope.

14. A fiber amplifier system comprising:a splitter splitting a feedback
beam into a plurality of fiber beams;a plurality of polarization
controllers each receiving one of the fiber beams, said polarization
controllers providing polarization orientation control;a plurality of
fiber amplifiers each receiving and amplifying a fiber beam, said fiber
amplifiers each including an output end;a tapered fiber bundle including
an input end and an output end, said input end being coupled to the
output end of all of the fiber amplifiers, said output end of the tapered
fiber bundle being a combined fiber including a portion of all of the
fiber amplifiers with fiber cores in a single fiber mass, said tapered
fiber bundle outputting a combined output beam;an end cap optically
coupled to the output end of the tapered fiber bundle, said end cap
expanding the output beam from the tapered fiber bundle;a collimating and
magnifying telescope receiving the output beam from the end cap, said
collimating and magnifying telescope expanding and collimating the output
beam;a beam sampler for sampling a portion of the output beam from the
collimating and magnifying telescope and providing a sample beam;a
polarization detector detecting the polarization in the fiber beams in
the sample beam and providing polarization measurement error signals;a
synchronous polarization processor receiving the polarization measurement
error signals and controlling the polarization controllers to control the
orientation of the polarization of the fiber beams in the fiber
amplifiers in response to the polarization measurement signals; anda
single mode fiber receiving the sample beam from the beam sampler, said
single mode fiber providing the feedback beam.

15. The system according to claim 14 wherein the fiber amplifiers are
coupled together into a multi-core fiber where each fiber in the
multi-core fiber includes an air cladding layer, said multi-core fiber
being chemically etched at one end to separate constituent fibers, and
wherein the individual fiber amplifiers are coupled to the multi-core
fiber by splices to said constituent fibers, and the other end of said
multi-core fiber is tapered to form to the tapered fiber bundle.

16. The system according to claim 14 wherein the end cap includes a
negative gradient index lens and a uniform glass rod optically coupled
together where the negative gradient index lens is coupled to the output
end of the tapered fiber bundle.

17. The system according to claim 14 wherein the end cap includes a
positive gradient index lens coupled to an output end of the end cap or a
positive lens formed by a curved surface at an output end of the end cap.

18. A fiber amplifier comprising:a splitter splitting a feedback beam into
a plurality of fiber beams;a plurality of fiber amplifiers each receiving
and amplifying a fiber beam, said fiber amplifiers each including an
output end;a tapered fiber bundle including an input end and an output
end, said input end being coupled to the output end of all of the fiber
amplifiers, said output end of the tapered fiber bundle being a combined
fiber including a portion of all of the fiber amplifiers with fiber cores
in a single fiber mass, said tapered fiber bundle outputting a combined
output beam;an end cap optically coupled to the output end of the tapered
fiber bundle, said end cap expanding the output beam from the tapered
fiber bundle;a combined lens and sampling grating assembly including a
lens and a sampling grating, said sampling grating providing a sample
beam from the output beam from the end cap, said lens collimating the
output beam from the end cap; anda single mode fiber receiving the sample
beam from the sampling grating, said single mode fiber providing the
feedback beam.

19. The system according to claim 18 further comprising a polarization
detector, a synchronous polarization processor and a plurality of
polarization controllers each receiving one of the fiber beams, said
polarization detector detecting the polarization of the fiber beams in
the sampled beam and providing polarization measurement error signals to
the polarization processor, said polarization processor controlling the
polarization controllers to control the polarization of the fiber beams
in the fiber amplifiers in response to the polarization measurement error
signals to make the orientation of the polarization of the fiber beams
same.

20. The system according to claim 19 wherein the polarization detector
detects the polarization of the fiber beams by detecting a unique dither
on the fiber beams in phase or amplitude using distinct frequencies for
frequency modulation or amplitude modulation or a distinct code for code
division multiple access or time division multiple access.

Description:

BACKGROUND 1. Field of the Disclosure

[0001]This disclosure relates generally to a high power fiber laser
amplifier and, more particularly, to a high power fiber laser amplifier
that couples ends of the fiber amplifiers into a tapered fiber bundle to
combine the beams with improved fill factor.

[0002]2. Discussion of the Related Art

[0003]High power laser amplifiers have many applications, including
industrial, commercial, military, etc. Designers of laser amplifiers are
continuously investigating ways to increase the power of the laser
amplifier for these applications. One known type of laser amplifier is a
fiber laser amplifier that employs doped fibers and pump beams to
generate the laser beam. Typically, a high power fiber laser amplifier
uses a fiber that has an active core diameter of about 10-20 μm or
larger. Modern fiber laser amplifier designs have achieved single fiber
power levels up to 5 kW. Some fiber laser systems employ multiple fiber
laser amplifiers and combine them in some fashion to higher powers.

[0004]A design challenge for fiber laser amplifiers is to combine the
beams from each fiber in a coherent manner so that the beams provide a
single beam output having a uniform phase over the beam diameter such
that the beam can be focused to a small focal spot. Focusing the combined
beam to a small spot at a long distance (far-field) defines the beam
quality of the beam, where the more coherent the individual fiber beams
the more uniform the combined phase and better the beam quality.
Improvements in fiber laser amplifier designs increase the output power
and coherency of the fiber beams in such a way as to approach the
theoretical power and beam quality limit of the laser system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005]FIG. 1 is a schematic plan view of a known fiber laser amplifier
including a fiber lens array;

[0006]FIG. 2 is a cross-sectional view of the fiber lens array used in the
fiber amplifier shown in FIG. 1;

[0007]FIG. 3 is a schematic plan view of a known fiber laser amplifier
including a DOE combiner;

[0008]FIG. 4 is a schematic plan view of a fiber laser amplifier including
a tapered fiber bundle and a beam phase detector;

[0009]FIG. 5 is a diagram of a tapered fiber bundle and an end cap;

[0010]FIG. 6 is a cross-sectional view of an input end of the tapered
fiber bundle shown in FIG. 5;

[0011]FIG. 7 is a cross-sectional view of an output end of the tapered
fiber bundle shown in FIG. 5;

[0012]FIG. 8 is a profile of the near-field beam intensity of an output
beam from the tapered fiber bundle shown in FIG. 5;

[0013]FIG. 9 is a graph with core diameter on the horizontal axis and
effective mode diameter on the vertical axis showing the effective
diameter of the mode of a step index fiber;

[0014]FIG. 10 is a profile of a near-field beam intensity distribution of
a closely packed seven fiber bundle before being tapered;

[0015]FIG. 11 is a profile of a near-field beam intensity distribution of
the seven fiber bundle shown in FIG. 10 after being tapered;

[0016]FIG. 12 is a profile of a near-field beam intensity distribution of
a closely packed nineteen fiber bundle;

[0017]FIG. 13 is a cross-sectional view of an input end of a tapered fiber
bundle including a low index glass cladding;

[0018]FIG. 14 is a cross-sectional view of an output end of the tapered
fiber bundle shown in FIG. 13 including the low index glass cladding;

[0019]FIG. 15 is a perspective view of an end cap for a tapered fiber
bundle including a negative GRIN lens;

[0020]FIG. 16 is a perspective view of a segmented end cap for a tapered
fiber bundle;

[0021]FIG. 17 is a perspective view of a tapered end cap for a tapered
fiber bundle;

[0022]FIG. 18 is a perspective view of a segmented end cap for a tapered
fiber bundle including a positive GRIN lens;

[0023]FIG. 19 is a schematic plan view of a fiber laser amplifier
including a tapered fiber bundle, a phase detector and fiber polarization
controllers;

[0024]FIG. 20 is a cross-sectional view of a multi-core fiber;

[0025]FIG. 21 is an illustration of the multi-core fiber shown in FIG. 20;

[0026]FIG. 22 is a schematic plan view of a fiber laser amplifier
including a plurality of master oscillators, tapered fiber bundles and
phase detectors;

[0027]FIG. 23 is a schematic plan view of a fiber laser amplifier
including a plurality of master oscillators, an SBC grating and a
plurality of phase detectors;

[0028]FIG. 24 is a schematic plan view of a fiber laser amplifier
including a plurality of master oscillators, an SBC grating, phase
detectors and fiber polarization controllers;

[0029]FIG. 25 is a schematic plan view of a fiber laser amplifier
including a plurality of master oscillators, a plurality of
pre-dispersion gratings and an SBC grating;

[0030]FIG. 26 is a schematic plan view of a fiber laser amplifier
including a plurality of master oscillators and an SBC grating a
staircase mirror;

[0031]FIG. 27 is a schematic plan view of a known fiber laser amplifier
including a feedback single mode fiber and a pre-amplifier;

[0032]FIG. 28 is a schematic plan view of a fiber laser amplifier
including a tapered fiber bundle, a feedback single mode fiber, a
pre-amplifier and a beam sampler;

[0033]FIG. 29 is a schematic plan view of a fiber laser amplifier
including a tapered fiber bundle, a feedback single mode fiber, a
pre-amplifier and fiber polarization controllers; and

[0034]FIG. 30 is a schematic plan view of a fiber laser amplifier
including a tapered fiber bundle, a feedback single mode fiber, a
pre-amplifier and a sampling grating.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035]The following discussion of the embodiments of the disclosure
directed to fiber laser amplifiers including tapered fiber bundles is
merely exemplary in nature, and is in no way intended to limit the
invention or its applications or uses.

[0036]FIG. 1 is a schematic plan view of a known fiber laser amplifier
system 10 including a master oscillator (MO) 12 that generates a signal
beam on optical fiber 14. A fiber laser amplifier system of the type
shown in FIG. 1 can be found in U.S. Pat. No. 6,708,003 issued Mar. 16,
2004 to Wickham et al., titled Optical Energy Transmission System
Utilizing Precise Phase and Amplitude Control, assigned to the assignee
of this application and herein incorporated by reference. The signal beam
is spilt into a certain number of split beams by a splitter and phase
modulators 16, where a separate phase modulator 16 is provided for each
split beam. The splitter and the phase modulator are actually two
separate devices, but shown here is a single object because they can be
implemented on a single chip. The phase modulators 16 adjust the phase of
each split beam so that all of the beams are in phase with each other in
a coupled output beam 26, as will be discussed in further detail below.
The split beams from the phase modulators 16 are then sent to fiber
amplifiers 18 where amplifiers 20 represent the doped amplifying portion
of the fiber amplifiers 18 that receive an optical pump beam (not shown).
The amplified fiber beams from the fiber amplifiers 18 are then sent to a
fiber lens array 22 including a cylindrical fiber lens 24 for each fiber
amplifier 18, where each of the lenses 24 are coupled together as the
array 22 so that all of the fiber beams are be coupled together as the
coupled output beam 26. The fiber lens array 22 collimates and precisely
co-aligns each of the fiber beams to form a tiled array of collimated
beams. The coupled output beam 26 is sent to a beam sampler 28 that
splits the beam 26, where the majority of the beam intensity is provided
as the output beam of the system 10.

[0037]The MO 12 also provides a reference beam on a fiber 30 that is
amplified by a fiber amplifier 32 and collimated by a lens 34. The
collimated reference beam from the lens 34 is sent to the beam sampler 28
where the reference beam interferes with each of the fiber beams in the
coupled beam 26 to provide an interference pattern between the reference
beam and each separate fiber beam. The interfered beams are directed by
lenses 36 to an array of phase detectors 38, where a separate phase
detector 38 is provided for each separate fiber beam. An electrical
signal defining the interference pattern between the beams from the
detectors 38 is sent to a phase processor and controller 40 that provides
phase correction signals to each of the phase modulators 16 to adjust the
phase of the split beams from the MO 12 so that they are all in phase
with each other and the output beam 26 is one coherent beam that can be
tightly focused in the far-field.

[0038]FIG. 2 is a cross-sectional view of the lens array 22 including the
individual lenses 24. As is apparent from this configuration, the
cylindrical shape of the lenses 24 creates a dead space 42 between the
lenses 24, which results in a reduced fill factor, defined as the
fraction of the combined beam area occupied by the high power beams. As
used herein, improved fill factor means a larger fill factor and better
beam quality or focusability to a smaller diffraction limited spot. By
making the beams in phase with each other and continguous, the beam
quality of the output beam 26 is improved and it can be focused to a
small spot. Therefore, it is desirable to make the lenses 24 as tightly
packed together as possible. Further, the actual beam propagating through
the core in each of the fibers are Gaussian beams that have a bell shape
beam profile with a higher center intensity, and reduced peripheral
intensity. When a close packed array of Gaussian beams is focused, the
central focal lobe will typically include only about 60% of the combined
beam power as a result of the Gaussian shape and intervening dead space
between beams. Thus, the reduced fill factor of the combined beam array
results from a combination of both the Gaussian shape of the individual
beams and the intervening dead space 42, where the combined output power
being focused in the central far-field focal lobe is given by the fill
factor, which is about 60% of the total beam power.

[0039]FIG. 3 is a schematic plan view of a known fiber laser amplifier
system 50 that eliminates the fill factor discussed above, where like
elements to the system 10 are identified by the same reference numeral. A
fiber amplifier of this type can be found in U.S. Pat. No. 7,440,174
issued Oct. 21, 2008 to Rice et al., titled Coherent Fiber Diffractive
Optical Element Beam Combiner, assigned to the assignee of the present
application and herein incorporated by reference. In this embodiment, the
fiber amplifiers 18 are spliced into a single fiber array 52 to generate
an array of closely spaced output beams 54. The output beams 54 are
collimated by optic 56 and then sent to a diffractive optical element
(DOE) 58 that combines the beams when they are precisely aligned and
phased. The diffracted beams from the DOE 58 provided at the same angle
are directed to a beam sampler 60 that splits the beams where a majority
portion of the combined beam is the output beam from the system 50.

[0040]A split portion of the combined beam from the DOE 58 is collected by
focusing optics 52 and sent to a phase detector 64. The phase detector 64
measures the phase of the combined beam and sends a measurement signal to
a synchronous phase processor 66. By detecting the phase of the combined
beams in this manner, the reference beam can be eliminated and a single
phase detector can be employed. The phase of the constituent beams can be
distinguished in the single output phase detector 64 by uniquely
dithering or coding the constituent fiber beams in phase or amplitude,
such as by using distinct frequencies for frequency modulation (FM) or
amplitude modulation (AM), distinct codes for code division multiple
access (CDMA) or time division multiple access (TDMA), etc., so that a
synchronous detector scheme can distinguish the constituent phase signals
for each fiber beam in the combined beam. Such a technique is disclosed
in U.S. Pat. No. 7,346,085 issued Mar. 18, 2008 to Rothenberg et al.,
titled Multi-Stage Method and System for Coherent Diffractive Beam
Combining, assigned to the assignee of this application and herein
incorporated by a reference. The synchronous phase processor 66 decodes
the distinct constituent phases in the measurement signal form the phase
detector 64, and generates phase error correction signals for each fiber
beam that are sent to the corresponding phase modulators 16 so that
adjustments to the phase of the individual fiber beams in the fiber
amplifiers 18 causes all of the constituent fiber beams in the output
beam to be locked in phase. Because the array of fiber beams 52 is
combined into a single output beam, the fill factor problem is
eliminated, and the output beam can be focused to a nearly diffraction
limited spot to reach nearly the theoretical limit of brightness provided
by the total combined power of the beams.

[0041]Diffracted beams 68 from the DOE 58 other than the combined output
beam have limited angular separation, and thus require a fairly large
path length to sufficiently separate the diffracted output beams, thus
making the system 50 less compact. Further, the array of output fibers
must be aligned to very high precision with each other and to the output
optics of the system 50 in order to achieve high beam combination
efficiency. Such precision alignment is even more challenging in the
presence of unavoidable thermal dissipation that accompanies the presence
of multi-kW laser beams. Thus, it would be desirable to provide a fiber
amplifier system having excellent beam quality, but avoids the need for
fiber arrays to be precisely aligned to bulky free space optical
elements. In addition, an approach that combines the beams in an
all-fiber format can provide an ideal packaging solution for power
scaling at high power within a single fiber aperture, which can then be
either directly injected into a telescope or used as a building block for
further beam combinations.

[0042]FIG. 4 is a schematic plan view of a fiber laser amplifier system 70
that provides improvements over the systems 10 and 50 discussed above by
providing beam combining with higher fill factor and beam quality in the
fiber material itself. In the system 70, like elements to the systems 10
and 50 are identified by the same reference number. In this embodiment,
ends of the fiber amplifiers 18 are joined to an input end of a tapered
fiber bundle 72 that combines the fiber amplifiers 18 into a single fiber
mass. An end cap 74 is then mounted to an output end of the tapered fiber
bundle 72. The output beam from the end cap 74 is collected and focused
by a telescope 76 including receiving optics 78 and collimating optics
80. The beam from the telescope 76 is sampled by a beam sampler 82 where
the majority of the beam is directed out of the system 70 as an output
beam. In the manner as discussed above, the sampled portion of the
combined beam from the beam sampler 82 is focused by focusing optics 84
onto a phase detector 86 that measures the phase of the combined beam and
sends an electrical signal of the phase measurement to a synchronous
N-beam phase processor 88. The processor 88 then sends phase error
correction signals to the phase modulators 16 to control the phase of the
beams in the fiber amplifiers 18 so that all of the constituent fiber
beams in the combined output beam are locked together with the same phase
in the manner as discussed above. Similar to the method described in the
laser system 50, in order to be able to determine the proper phase
control signals for the individual beams that are split by the splitter
16, the phase modulators 16 apply a distinct dither frequency for FM or
AM, or a distinct code for CDMA or TDMA, on each split beam that are in
the combined beam at the output of the system 70. The phase detector 86
can detect the distinct dither frequencies or codes, and the processor 88
can use that information to determine a phase error for each split beam,
and provide error correction signals to the corresponding phase
modulators 16 for each of the respective split beams to properly phase
lock all of the constituent beams in the combined output.

[0043]FIG. 5 is an illustration of seven fibers 100, each having an outer
cladding layer 102 and an inner core 104 through which the beam
propagates, being coupled to a tapered fiber bundle 106 of the type
referred to above. FIG. 6 shows a cross-sectional view of an input end of
the tapered fiber bundle 106 with the seven fibers 100 at an inner
portion of the bundle 106 and a plurality of cladding fibers 108 formed
around the bundle of fibers 100. FIG. 7 is a cross-sectional view of an
output end of the tapered fiber bundle 106 showing that the combination
of the fibers 100 and the cladding fibers 108 has been formed into a
single fiber mass 110 where points 112 represent the cores 104 of the
fibers 100. FIG. 8 is a cross-sectional view of a beam profile 116 in the
near-field of the beam that is output from the end cap 114.

[0044]The tapered fiber bundle 106 can be made by any of the well known
technique for fabricating tapered fiber bundles where the fibers 100 and
108 are gathered in a bundle, and the bundle is then drawn down in
diameter in a carefully controlled high temperature fusion process. The
end result is a scaled down version of the initial closely packed bundle
of fiber amplifiers where the final core diameter 2a and the spacing b
determines the final output fill factor of the combined beam. Because
these beams are of very high intensity it is necessary to splice an end
cap 114 to the output end of the tapered fiber bundle 106 to avoid damage
at the exit surface of the tapered fiber bundle 106. The combined beam
expands by diffraction in the end cap 114 until the peak intensity is
sufficiently reduced so that surface damage is avoided. A gradient index
(GRIN) lens with a negative focal length can be incorporated into the end
cap 114 to increase the divergence of the output beam, as will be
discussed below.

[0045]Once the output beam exits the end cap 114 it is collimated and/or
imaged by a simple lens or curved mirror to a desired beam size and
collimation by the telescope 76. There is no need for a lens array or
other precise fiber-to-fiber alignment. The external optics are simply
collimation and/or telescope optics used to magnify the beam to a desired
size, which are commonly used in many high power laser systems and beam
directors. This is in contrast to the systems 10 and 50 which both
require a very precise alignment of each of the individual fibers to
external free-space optics. There is no such requirement in the system 70
beyond the usual alignment requirements of the entire single beam in the
final telescope. In addition, there is no requirement on the exact
spacing of the fiber cores within the tapered fiber bundle 72, other than
to space them as close as possible, and the co-alignment of the cores is
quite relaxed because the divergence of each individual fiber is 10's of
mrad. Thus, this approach provides a combined output beam with N times
the beam power out of a single fiber aperture and a minimum of free-space
optics, where N is the number of fiber beams combined. The laser system
70 thereby provides a quantum leap in integration, compactness and
ruggedness in comparison to the systems 10 and 50.

[0046]The tapered fiber bundle 106 maximizes the fill factor by bringing
the fiber cores close together so that the individual fiber modes
overlap. Once the modes overlap, there will be cross-coupling and
interference between the fiber modes. By locking the phases of the fibers
together, as discussed above, formation of an in-phase super-mode can be
ensured, which exhibits constructive interference between all of the
fibers and significantly enhances the intensity in the gaps between the
beams. In this way, it can achieve a combined beam with a continuous
intensity profile and little or no intervening dead space. The challenge
is making the tapered fiber bundle to ensure there is negligible loss
within the bundle. Thus, the input fibers to the tapered fiber bundle 106
must have a sufficiently large diameter cladding so that very little
power appears at the cladding surface. Generally, this will require a
cladding diameter b to be about 2-3 times the core diameter 2a, which,
for the large mode area fibers of interest, limits the power at the
cladding surface to 1 PPM of the total. Since the cores are separated by
the cladding diameter b, this ratio will predominately determine the
pre-tapered fill factor. The fill factor can be quite low for a ratio of
b/2a=3, where only about 20% of the power is focused into the central
lobe with 25 μm cores and NA=0.06, where NA is the numerical aperture.
As the bundle is tapered down, both the core and cladding diameters will
generally decrease in proportion as the fibers also fuse together so that
this ratio of cladding-to-core diameter is approximately maintained
through the taper, and hence, it would appear that the fill factor is
unchanged. In addition, tapering down the core diameter would appear to
reduce the mode diameter such that the peak intensity increases, which
may be limiting for very high power amplifiers. However, the surprising
result is that as the core diameter decreases during the taper, the mode
shape changes such that the mode area reduction is limited to a minimum
value and the tails of the mode field distribution broaden significantly.
This behavior serves to both limit the peak intensity within the tapered
fiber bundle 72 and ensures better overlap of the modes, and thus, rather
that remaining constant through the taper, the fill factor can increase
significantly.

[0047]FIG. 9 is a graph with core diameter on the horizontal axis and
effective mode diameter on the vertical axis that shows the effective
diameter of the mode of a step index fiber with NA=0.06 as a function of
the core diameter. The effective diameter in this plot is defined as a
1/e2 intensity diameter of a Gaussian that has the same peak
intensity as the fiber mode. It can be seen that as the core diameter is
decreased, the mode diameter reaches a minimum of approximately 13 μm,
and then rapidly increases. It should be noted that the fiber is strictly
single mode when the core diameter is less than about 14 μm
(V#=NA×2πa/λ<2.4, for λ=1080 nm). This rapid
increase of the mode diameter for smaller cores is the result of the
increase in the tails of the mode. Starting with an initial core diameter
of 25 μm, it can be shown that the mode is well confined within a
negligible power beyond approximately 2.5 times the core diameter, but as
the core diameter, and thus the V#, decreases, the reduced confinement of
the mode tails increases the effective mode diameter, and thus increases
the mode overlap in the tapered fiber bundle 72. With a further reduction
in the core diameter, as the V# approaches approximately 1, the tails
approach very limited confinement, and thus allow the arbitrarily large
mode overlap, but also for increased losses out of the cladding layer. If
the phases of the individual beams are locked to ensure in-phase
(constructive) interference in the forming super-mode, then the fill
factor penalty can be greatly reduced. By optimizing the core size at the
output of the tapered fiber bundle 72 good overlap can be achieved, while
still allowing excellent confinement within the now larger cladding of a
reasonably sized bundle, such as 400 μm diameter.

[0048]It should be noted that the process can be improved beyond a simple
tapering process in which the core size and spacing both decrease in
proportion. The use of carefully tailored temperature in the tapering
process can lead to enhanced diffusion of dopants around the core, and
therefore the effective core size can be increased by diffusion relative
to the proportionate change in the core-to-core spacing. This process
effect can further enhance the tapered fiber bundle output mode fill
factor.

[0049]As an example of a combined output beam obtained from the end cap
74, consider a hexagonally closely packed tapered fiber bundle that takes
seven fibers with 25 μm/62.5 μm core/cladding diameters as an
input, where the initial core-to-core spacing is also about 62.5 μm.
The input is tapered down to about 3.6 times to a 6.9 μm core size,
where the V# is 1.2, and the core-to-core spacing is reduced to 17.2
μm. The input fiber modes have a negligible fraction (approximation
signal 1 PPM) of the fiber power at the untapered cladding interface, but
the modes have a large overlap with the neighboring cores once they are
fused together and tapered down. Propagation simulations show that proper
adiabatic tapering of the cores limits out-coupling from the lowest order
mode in each core to 10's of PPM. All of the mode fields are assumed to
have been phased so that they add coherently, and thus fully maximize the
fill factor.

[0050]A near-field intensity distribution 120 of a closely packed seven
fiber bundle with 25 μm/62.5 μm core/cladding diameters before
being tapered is shown in FIG. 10 where ring 122 is the assumed reference
aperture diameter Dref of about 190 μm, which is used to define
the far-field diffraction limited radius λ/Dref. It can be
shown that the LMA modes are well confined and do not overlap, and
because of the large spacing between the input cores, the fill factor is
quite low. It can be further shown that the calculated power in the
bucket (PIB) of a combined beam based on this geometry is only about 17%
within the diffraction limited far-view of angular radius
1.2λ/Dref. In comparison, a diffraction limited flat top beam
that fully fills the reference aperture achieves about an 84% PIB in this
diffraction limited angular bucket.

[0051]Tapering down this seven fiber input bundle to an assumed 6.9 μm
core diameter in a 17.2 μm core-core spacing yields a very different
combined output beam as shown by the near-field intensity distribution
124 in FIG. 11, where ring 126 is the assumed reference diameter. The
near-field reference aperture diameter in this case is chosen to be 69
μm, which contains greater than 99% of the combined power. The
combined tapered fiber bundle output, because of the greatly increased
mode overlap and fill factor, now has a very high efficiency of focus
into a diffraction limited far-field bucket. It can be shown that the PIB
of seven ideally phased beams is about 92% into the diffraction limited
angular radius 1.2λ/Dref. Note that this PIB exceeds the 84%
achieved by a diffraction limited and fully filled flat top beam. The PIB
of the seven combined beams increases to about 95% within a radius of
1.5λ/Dref. Therefore, it can be shown that the effect of the
tapered fiber bundle 72 is to dramatically increase the fill factor and
PIB compared with the input fiber bundle.

[0052]For a given core geometry, an effective area of the combined beam
can be defined based on the peak intensity, which can be used to define a
maximum power before intensity driven damage becomes an issue, where the
peak intensity of the combined beam of power P is defined as
Ipeak=P/Aeff. For the seven beam combination with the tapered
fiber bundle discussed above, Aeff=630 μm2, whereas a single
constituent beam at the tapered fiber bundle output has an effective area
of 80 μm2, and hence the effective area is increased by 7.8 times
over a single beam.

[0053]For higher power, a larger number of input fibers can be employed.
For hexagonal close packing, the next magic number with an additional
ring of fibers is nineteen, which, based on the above core diameters of
spacing, yields an effective area of about 1860 μm2, and thus
would enable more than 60 kW in a single tapered fiber bundle output,
assuming ˜3 kW per input beam. A near-field intensity distribution
130 of an output of a tapered fiber bundle with a ring aperture 132 is
shown in FIG. 12, where the aperture reference diameter is 96 μm.

[0054]As described above, it has been assumed that the super-mode formed
in simply the coherent in-phase super position of the individual fiber
modes. By symmetry, if the six outer beams have phases locked and equal,
then there are just two modes of interest, where the central beam is
either in phase or out of phase, referred to as the in-phase |+>and
out-of-phase |->super-modes, respectively. Therefore, the above
results depend on suppression of the out-of-phase |->super-mode by
proper phasing of the input beams. The use of the phase-locking systems
can certainly ensure that the central beam at the output has the proper
relative phase with respect to the outer beams. However, because of the
large mode overlap between the constituent fibers and the tapered fiber
bundle 72, there is considerable power exchange between the cores.
Simulations show that for the above example core diameter at the end of
the taper, power launched in the central core would couple from the
central beam to the outer beams in about a 2.5 mm propagation distance.
Therefore, to ensure the desired uniform power distribution of the beams,
besides proper phasing of the input fields, the length and taper of the
taper fiber bundle 72 must be tailored. In fact, the central peak in the
assumed output beam has about 30% higher power than the outer peaks of
the beam. Hence, by designing the length of the tapered fiber bundle 72
so that the power coupling between cores reduces the central core power
somewhat, the peaks can be evened out and a reduction in the peak
intensity for a given total array power can be provided, thereby
increasing the total power limit for a given damaged threshold. The
required design accuracy for a few percent power balance, based on the
simulated 2.5 mm coupling distance is a few 100 microns, which should be
easily achieved.

[0055]Current commercial high power tapered fiber bundle packages have
dissipation capabilities of about 100 W, and this is likely to grow as
development of these devices continues. Reports of multi-mode pump
couplers used for fiber amplifiers combining over 1 kW is routine with
the pump throughputs achieved at greater than 98%. These commercial
devices generally attempt to maximize pump brightness by coupling a
tapered fiber bundle to an output fiber with an angular acceptance only
slightly larger than the effective cumulative acceptance of the input.
Therefore, these devices generally have a significant, i.e., greater than
1%, coupling losses. In the type of tapered fiber bundle proposed here,
there is no loss from coupling to an output fiber because only an end cap
is employed. The intrinsic absorption losses of high quality transmission
fibers that is used in the tapered fiber bundle is very low, i.e., less
than 10 PPM/cm, and therefore is not expected to be a limiting factor.

[0056]The remaining losses result from large angle mode conversion and
scattering during propagation or near the end of the taper of the tapered
fiber bundle. This will of course depend on the design and quality of
fabrication of the tapered fiber bundle. However, the LMA input fibers of
interest have quite a low NA, i.e., approximately 0.06, and the angular
divergence of this input light is limited even including the residual
power in the mode wings. For example, a 25 μm/0.06 NA LMA fiber mode
has less than 100 PPM residual power propagating at angles larger than
about ±10 mrad. Even the mode of the small 6.9 μm core at the end
of the tapered fiber bundle described above has less than 100 PPM of
residual power outside angles of ±0.2 rad. Heating within the tapered
fiber bundle package is likely to be dominated by large angle out-coupled
light that is absorbed by tapered fiber bundle cladding materials.
Therefore, the use of a moderate NA glass cladding material in the
tapered fiber bundle, which is virtually non-absorbing, should greatly
mitigate heating from all but very large angles scattering within the
tapered fiber bundle. For example, fluorine-doped glasses can be used as
a cladding material with an NA limit of approximately 0.3, and thus can
confine any lower angle scattered light to prevent absorption into the
tapered fiber bundle package and allow escape through the end cap.

[0057]FIG. 13 is a cross-sectional view of an input end of a tapered fiber
bundle 140 including an outer low index of refraction glass tube 142, and
FIG. 14 is a cross-sectional view of an output end of the tapered fiber
bundle 140 showing the glass tube 142, as discussed above.

[0058]As discussed above, the end cap 74 is used to get the combined high
power beam out of the glass without damage or degradation of beam
quality. As discussed, the purpose of the end cap 74 is to allow the beam
to expand sufficiently so that the intensity at the exit surface is below
the damaged threshold. Secondly, it must be ensured that the power
reflected from that surface does not adversely affect the fiber amplifier
performance. Therefore, it is typically preferable to provide an
anti-reflective (AR) coating on the output face of the end cap 74 to
minimize reflections. For the small beams being discussed herein, it has
been reported that damage thresholds greater than about 1 MW/cm2 are
achievable. For a 20 kW output beam, this implies the beam must expand to
an effective area of about 2 mm2. For the seven beam combination
discussed above, the effective 1/e2 diameter of the combined beam as
it enters the end cap 74 is about 45 μm, and the aggregate divergence
angle is thus quite small, i.e., the angle is approximately ±0.01 at
1/e2 in glass, so that a long propagation distance is required to
reduce the peak intensity. Calculations show that the peak intensity is
reduced to about 1 MW/cm2 for a 20 kW seven beam output after
propagation of about 11 cm at which point the beam is roughly Gaussian
with an FWHM of approximately 1.3 mm. Therefore, the lens cap diameter
will need to be increased to about 5 mm either in a tapered fashion or in
segments to accommodate the expanding beam at the output facet, as will
be discussed below.

[0059]Even with the very low absorption end caps, the long propagation
distance in glass poses a difficulty from accumulated thermal optical
path distortion (OPD). However, this is mitigated by the high aspect
ratio of the end cap 74, since the beam is at its largest at about 1 mm.
Surface cooling of the end cap 74 should be adequate, but there will
still be an unavoidable quadratic temperature variation because of the
intrinsic absorption in the end cap 74. Approximating the heat deposition
as uniform over the extent of the beam, the temperature difference
induced by the absorption over the beam width is approximately
ΔT=Pα/4 πk=Pα/180° C., where P is the total
beam power in kW, α is the intrinsic glass absorption in PPM/cm,
and the glass conductivity is κ=1.4 W/m-° C. The OPD in
glass is about 1.3 waves per cm of length and ° C. of temperature
difference, and therefore for a 20 kW beam and 10 cm path length, the
maximum OPD is about α/7 waves. Ultra-low absorption fused silica
has been reported with α<1 PPM/cm, so the OPD is not
overwhelming, and mostly spherical, however, this issue can present
serious limitations to power scaling with this method. This illustrates
that thermal management of the end cap 74 for fiber schemes that operate
at 10+ kW power levels will be quite important for minimizing OPD.

[0060]As the number of beams scales up, this issue is exacerbated because
the combined beam has a larger effective diameter, i.e., about 70 μm,
and thus, even smaller divergence. For the nineteen beam combination
discussed above, the calculated divergence angle at 1/e2 is about
±7.2 mrad in glass, and combined with the larger 60 kW total power,
would require about a 27 cm long end cap to reduce the exit intensity to
about 1 MW/cm2. The predominate problem is the very small divergence
of the combined beam.

[0061]One approach to mitigating this issue is to fabricate an end cap
that includes a negative gradient index (GRIN) lens close to the tapered
fiber bundle splice. FIG. 15 is a perspective view of an end cap 150
including a negative GRIN lens 152 that is coupled to the tapered fiber
bundle. The remaining portion of the end cap 150 is a uniform glass rod
154 where the GRIN lens 152 and the glass rod 154 are optically coupled
by a suitable splice 156. The negative focal length lens can increase the
divergence of the combined beam significantly, and thus, reduce the
required end cap length to a few cm, thereby greatly reducing the
accumulated OPD in the end cap 150. For example, a GRIN lens with a focal
length of -0.8 mm will increase the divergence of the seven beam tapered
fiber bundle output by roughly three times, and thus, reduce the OPD for
a 20 kW output beam proportionally to about α/20 waves. Such an
approach could make scaling of this scheme to single aperture powers
approaching 100 kW within reach.

[0062]The diameter of the end cap 74 could be increased in segments or by
a taper to accommodate the expanding beam. FIG. 16 is a perspective view
of an end cap 160 including stepped segments, where a negative GRIN lens
162 is coupled to the tapered fiber bundle and to a uniform glass rod 164
of about the same diameter by a splice 168. An opposite end of the glass
rod 164 is spliced to a larger diameter glass rod 166, which in turn is
spliced to an even larger diameter glass rod 170 to provide the segments
for the beam expansion. An anti-reflective coating 172 can be provided on
an output surface of the glass rod 170 opposite to the GRIN lens 162.

[0063]FIG. 17 is a perspective view of an end cap 180 including a negative
GRIN lens 182 to be coupled to the tapered fiber bundle at one end and
coupled to a uniform glass rod 184 of about the same diameter by a splice
186 at an opposite end. A tapered glass rod 188 is then coupled to the
uniform glass rod 184 where a wide end of the tapered glass rod 188
includes an anti-reflective coating 190.

[0064]With a standard AR coating reflectivity of 0.2%, the reflected power
for 20 kW is only 40 W in an expanded beam, so the fraction of this
reflection that re-enters the small tapered fiber bundle output fiber
should be straight forward to be limited to small and safe powers.

[0065]For a large aperture beam director, it would be desirable that the
magnified image of the tapered fiber bundle near-field be relayed to the
beam director aperture. This is accomplished by the telescope 76 where
the lens 78 has a focal length f1 and the lens 80 has a focal length
f2 and where the lenses 78 and 80 are separated by f1+f2,
which magnifies the image by length f2/f1.

[0066]It is possible to integrate the lens 78 into the end cap 74 by using
a spherical exit surface on the end cap 74 or by splicing a focusing GRIN
lens at the end cap output. FIG. 18 is a perspective view of an end cap
192 similar to the end cap 160, where like elements are identified by the
same reference numeral. The end cap 192 includes a positive GRIN lens 194
mounted to the anti-reflection coating 172 that operates as the lens 78.
Such optical arrangements can be integrated directly into the beam
director telescope as well. More compact telescopes of standard designs
for high magnification that use both positive and negative lenses can
also be implemented to optimize the footprint of the expansion optics.

[0067]In order to maintain proper beam quality, it is necessary that the
polarization of the fiber beams in each of the fiber amplifiers 18 have
the same orientation. For the system 70, the fibers employed in the fiber
amplifiers 18 are polarization maintaining fibers so that all of the
beams in all of the fibers have the same polarization orientation. In
certain applications, such as high power applications, it may not be
feasible to use polarization maintaining fibers, and thus, it becomes
necessary to align the polarization of each of the fiber beams in the
fiber amplifiers 18.

[0068]FIG. 19 is a schematic plan view of a fiber laser amplifier system
200 that does not employ polarization maintaining fibers, where like
elements to the system 70 are identified by the same reference number.
The system 200 uses a polarizer 202 to determined the polarization of the
fiber beams in the sampled beam from the sampler 82. As the polarization
in the fiber beams changes relative to each other, the polarizer 202
causes more or less light to be directed to a polarization detector 204.
The polarization detector 204 uses distinct frequency dithers or tags on
the individual beams to determine the polarization of each beam in the
output beam. The measurement of the polarization is provided to a
synchronous N-beam polarization processor 206 that determines the
relative orientation of the polarizations in the beams. The processor 206
uses the distinct dither frequencies or tags to identify the fibers for
all measured polarization changes and provides signals to polarization
controllers 208 for the corresponding fiber amplifiers 18 to control the
polarization orientation in each fiber so that they are the same. Such a
polarization controlling system has been proposed in U.S. Pat. No.
6,317,257, issued Nov. 13, 2001 to Upton et al., titled Technique for
Polarization Locking Optical Outputs, assigned to the assignee of this
application and herein incorporated by reference.

[0069]Forming the fiber amplifiers 18 into the tapered fiber bundle 72
provides a number of challenges. It is desirable to provide a certain
ratio of fiber core to fiber diameter and to provide the fiber cores as
closely spaced together as possible. Further, for fibers of the diameters
being discussed herein, the flexibility of the fibers limits the handling
ability of the fibers. Multi-core fibers are known in the art that
include multiple cores coupled together in a bundle surrounded by a
common cladding layer. Such a multi-core fiber would be easier to handle
and be formed into a tapered fiber bundle as discussed above. However, it
is then necessary to get the fiber beams into the individual cores within
the multi-fiber core. Further, it is known in the art to provide an outer
air cladding around the individual cores in the multi-core fiber to
provide high NA confinement of pump light within the cladding around each
core.

[0070]FIG. 20 is a cross-sectional view of a multi-core fiber 210 of the
type discussed above. The multi-core fiber 210 effectively includes a
plurality of individual fibers 212 each including a core 214 and an inner
cladding layer 216. Further, each individual core 214 and inner cladding
layer 216 is surrounded by an outer air cladding 222 that is formed by a
number of small glass air bridges 226 making the air cladding effectively
all air, in a manner that is well understood to those skilled in the art.
By providing the air cladding 222 around the individual cores 214, the
individual fibers 212 can be separated from a multi-core fiber body 224
by chemically etching the air bridges 226 within the air cladding 222 and
the glass in the multi-core fiber body 224.

[0071]FIG. 21 is a plan view of the multi-core fiber 210 where the
individual fibers 212 have been separated to form pigtails extending from
the multi-core fiber portion 218. In one embodiment, the multi-core fiber
body 224 and air claddings 222 are etched using hydrofluoric acid, or
another suitable chemical agent, to separate the individual fibers 212
from the portion 218 so that now the individual fibers 212 can be coupled
to the fiber amplifiers 18. Because the multi-core fiber portion 218 has
a significantly larger diameter than the individual fibers 212, it can be
more easily handled to form a tapered fiber bundle of the types discussed
above. It is noted that in the taper process, appropriately high
temperatures must be applied and perhaps a vacuum so that the bridges in
the air claddings 222 collapse so that the fiber cladding layers 216 are
continuous between the cores and the multi-core fiber body 224. This
enables the modes confined in each core 214 to spread and overlap with
the other modes in the tapered region of the multi-core fiber 210.

[0072]The embodiments discussed above can be extended to other types fiber
laser amplifier systems to further increase the output power of the
system. FIG. 22 is a schematic plan view of a fiber laser amplifier
system 230 that combines multiple beams using spectral beam combination
(SBC) to increase the beam power. In the system 230, a plurality of N
master oscillators 232 individually provide beams on fibers 234 that are
at different wavelengths (λ1, λ2, . . . ,
λN). Each master oscillator wavelength is then split into M
fibers by M splitters and phase modulators 236 in the manner as discussed
above. The separate fibers from each splitter and phase modulator 236 is
then coupled to a fiber amplifier 238 represented by amplifier 240. The
fiber amplifiers 238 are then coupled to a tapered fiber bundle 242,
which is coupled to an end cap 244 in the manner as discussed above. The
tapered fiber bundle 242 and the end cap 244 can be any of the tapered
fiber bundle and/or end cap embodiments discussed above.

[0073]The N tapered fiber bundles are arranged in a linear array, which is
placed at the back focal plane of common collimating optics 248. The
output from each end cap 244 is focused by a telescope lens 246 and the
combined beams for all of the master oscillator wavelengths are
collimated by the collimating optics 248. The collimated beams from the
collimating optics 248 are then sampled by a beam sampler 250 where most
of the beam is sent to an SBC grating 252. The SBC grating 252 is placed
in the opposing focal plane of the collimating optics 248 and its
dispersion along with the master oscillator wavelengths, spacing between
adjacent tapered fiber bundles and collimating optic focal length are
chosen so that each beam is precisely co-propagating with all of the
other beams after diffraction by the SBC grating 252. Thus, all of the
beams for each master oscillator wavelength are focused to the same spot
as all of the other master oscillator beam wavelengths.

[0074]The beam sampler 250 provides a small sample of the collection of N
beams incident on the grating 252, each of which is propagating at a
slightly different angle. Focusing optics 254 focuses the combined beam
onto N separate phase detectors, where each detector 256 measures the
phase relationship among the M beams at each separate master oscillator
wavelength. As above, a frequency tag is placed on each individual fiber
beam for each separate master oscillator wavelength so that the
measurement signals from the detectors 256 is received by a synchronous
phase processor 258 that adjusts the phase modulators 236 in each
wavelength group as discussed above. Thus, the signal from each of the N
phase detectors 256 is used to phase lock each group of M beams combined
by the tapered fiber bundle 242 at each of the N respective wavelengths.
The phase signal is synchronously processed to distinguish which of the M
fibers in a group the phase error originates and provides correction
signals to the appropriate modulators 236 so that the beams within each
wavelength group are optimally phase locked. In this embodiment, the
fiber amplifiers 238 are polarization maintaining fibers to ensure a
coherent and polarized output beam, and thus the highest possible
diffraction efficiently from the SBC grating 252 can be achieved, which
is typically much more efficient for one polarization state than the
other.

[0075]FIG. 23 is a schematic plan view of a fiber laser amplifier system
260 similar to the system 230, where like elements are identified by the
same reference number. The system 260 is a simplified design over the
system 230 that takes advantage of the zeroth order reflection from the
SBC grating 252. The first order reflection off of the SBC grating 252 is
the main beam focused to the desired location, where a partial portion of
the beam is reflected off the SBC grating 252 as the zeroth order.
Because the reflection of the zeroth order off the SBC grating 252 for
each separate wavelength group is slightly different, the focusing optics
254 can focus the separate beams onto the particular detector 256, as
discussed above. Thus, the system 260 does not need the beam sampler 250.

[0076]FIG. 24 is a schematic plan view of a fiber laser amplifier system
270 similar to the system 260, where like elements are identified by the
same reference number. The system 260 used polarization maintaining
fibers, which may or may not be feasible at high power. The system 270
does not employ polarization maintaining fibers in the fiber amplifiers
256, and thus a technique needs to be used to provide polarization
orientation between the fiber beams in each separate master oscillator
wavelength group. In order to do this, the system 270 employs a polarizer
272 between the focusing optics 254 and the detectors 256 that directs
part of the beams to N polarization detectors 274 that measure the
polarization of each separate wavelength group. The sampled beams may be
provided by the 0th order grating reflection shown in the system
270, or by a separate sampling optic as shown in the system 230. The
measured signals from the detectors 274 are provided to N polarization
processors 276 that determine the relative polarization orientation
between the M fiber beams in each of the N wavelength groups, and provide
a suitable signal to M polarization controllers 278 at the low power side
of each of the M fiber amplifiers 238.

[0077]The SBC grating 252 provides better beam quality and less divergence
if the beams from the master oscillators 232 have a very narrow beam
bandwidth. However, by providing a narrow beam bandwidth from the master
oscillator 232, acoustic affects within the various fibers and other
optical components cause stimulated Brillouin scattering (SBS) that tends
to damage optical components. Therefore, it is desirable to increase the
beam bandwidth of the master oscillator signals to prevent SBS, which
results in lower beam quality as mentioned.

[0078]FIG. 25 is a schematic plan view of a fiber laser amplifier system
280 that allows a wider beam bandwidth master oscillator, but provides a
narrower beam bandwidth at the SBC grating 25, where like elements to the
system 260 are identified by the same reference number. To provide this
feature, the system 280 includes N pre-dispersion gratings 282, one for
each wavelength group. The dispersion gratings 282 provide dispersion
compensation that has essentially the same dispersion as the SBC grating
252, but is oriented oppositely so as to cancel the net dispersion for
each wavelength group beam. The dispersion gratings 282 are oriented so
that the beams overlap on the SBC grating 252 and are incident at the
correct angle to provide co-propagation of the diffracted beams. The beam
quality is optimized when the beams from the dispersion gratings 282 are
imaged onto the SBC grating 252 using image relay telescopes 284. The
relay telescope optics may be cylindrical to allow for a large beam width
in a direction orthogonal to the dispersion direction so that the
intensity on the grating surface is maintained below the optical damage
threshold.

[0079]In the system 280, the dispersion gratings 282 must be individually
and precisely aligned with the SBC grating 252 in the manner discussed
above, which can be cumbersome and complex. FIG. 26 shows an alternate
embodiment for a fiber laser amplifier system 290 that helps with this
problem, where like elements to the system 280 are identified by the same
reference number. In the system 290, the individual dispersion gratings
282 are replaced with a single pre-dispersion grating 292 that operates
in the same manner. The individual beam wavelength groups are reflected
off of the pre-dispersion grating 292 at different angles, which need to
be corrected before they impinge the SBC grating 252 so that all the
beams are directed to the beam spot. A staircase mirror 294 is provided
having an individual stair step for each beam wavelength group, where the
steps are appropriately chosen to have step heights and widths to allow
the beams to have the proper angles so that all beams are co-aligned
after diffraction from the SBC grating 252. For high power applications,
cylindrical optics 296 and 298 are provided in the beam path between the
pre-dispersion grating 292 and the SBC grating 252 so as to spread the
power density of each of the beams to a line focus or a near-focus on a
different step of the staircase mirror 294 in order to limit the peak
intensity below the optics damage threshold. The pre-dispersion grating
properties and the incident angles are chosen to cancel the dispersion of
the SBC grating 252. One design with essentially no net dispersion is to
use identical gratings with opposite orientations for the pre-dispersion
and SBC gratings.

[0080]FIG. 27 is a schematic plan view of a known fiber laser amplifier
system 300, such as the type disclosed in U.S. Pat. No. 7,130,113, issued
Oct. 31, 2006 to Shakir et al., titled Passive Phasing of Fiber
Amplifiers, assigned to the assignee of this application and herein
incorporated by reference. The system 300 is different than the amplifier
system 10 and others described above because it does not employ a master
oscillator, but instead employs a light feedback loop. The amplifier
system 300 includes fiber amplifiers 302 represented by amplifiers 304
that are pumped by a pump beam (not shown) to generate the optical
amplification. The amplified signals from the fiber amplifiers 302 are
then sent to a lens array 306 of the type discussed above that collimates
the fiber beams. The individual lenses in the lens array 306 must be
precisely aligned so that all of the fiber beams are co-propagating in
the same direction. The co-propagating beam from the lens array 306 is
sampled by a beam sampler 308 where most of the beam passes through the
beam sampler 308 as the system output beam. The sampled portion of the
beam from the beam sampler 308 is focused by a coupling lens 310 and
collected by a collector 312 to be sent through a single mode fiber 314
that provides the beam feedback. Because the fiber 314 is single mode, it
passively provides the phase alignment of the fiber beams in the fiber
amplifiers 302, as opposed to the active controls provided by electrical
feedback to the phase modulators discussed above. An optical isolator 16
is provided in the single mode fiber 314 so that light only propagates in
the feedback direction. The feedback beam is amplified by a pre-amplifier
318 and split by a beam splitter to provide the fiber beams for the
several fiber amplifiers 302. This technique has been shown to be
effective in passively locking the phases of the fiber amplifiers 302,
but still suffers from the fill-factor problem discussed above with
reference to the system 10.

[0081]The system 300 can also be improved to be more compact in design and
reduce the optical components that require alignment by employing a
tapered fiber bundle in the same manner as discussed above. FIG. 28 is a
schematic plan view of a fiber laser amplifier system 330 showing this
embodiment, where like elements to the system 300 are identified by the
same reference numeral. The system 330 includes a tapered fiber bundle
332 that couples the fiber amplifiers 302 in the manner discussed above
to provide beam overlap at the output of the tapered fiber bundle 332. An
end cap 334 is coupled to the tapered fiber bundle 332, and can be any of
the various end cap embodiments discussed above. An output beam from the
end cap 334 is collected by a collimating and magnifying telescope 336
that includes focusing optics 338 and collimating optics 340. Thus, the
system 330 solves the fill factor problem of the system 300 in a compact
design. As above, the focusing optics 338 can be part of the end cap 334,
such as a positive GRIN lens.

[0082]It is possible that the systems 300 and 330 are passively
self-polarizing, meaning that all the fiber beams have the same
polarization state, which is required for coherent beam combination. This
can be done passively using the single mode fiber 314, or the
polarization of the fiber amplifiers 302 can be forced to all have the
same polarization by including polarization maintaining fibers.
Alternately, polarization controllers can be provided in the system to
maintain the polarization orientation in the fiber amplifiers 302 in the
manner as discussed above. FIG. 29 is a schematic plan view of a fiber
laser amplifier system 350 that provides polarization control, where like
elements to the systems 300 and 330 are identified by the same reference
numeral. In this embodiment, a polarizer 352 is provided between the
coupling lens 310 and the collector 312 that directs a portion of the
beam to a polarization detector 354 that measures the polarization
difference in the coupled beams from the output beam of the tapered fiber
bundle 332. A synchronous N-beam polarization processor 356 receives the
measured polarization signal from the polarization detector 354 and
controls polarization controllers 358 in each fiber amplifier 302 so that
the polarization orientation in each fiber amplifier 302 is maintained.
In order for the polarization processor 356 to identify which of the N
beams requires correction, each of the polarization controllers 358 must
provide a unique dither frequency or code, similar to the method
described for phase control in previous embodiments.

[0083]FIG. 30 is a schematic plan view of a fiber laser amplifier system
360 similar to the systems 300, 330 and 350, where like elements are
identified by the same reference numeral. In this embodiment, the
collimating and magnifying telescope includes a combined lens and
sampling grating assembly 362 including lens 364 and a sampling grating
366. The lens 364 collimates the output beam from the end cap 334 and the
sampling grating 366 redirects a small portion of the output beam onto
the coupling lens 310. The sampling grating 366 can provide an arbitrary
small sample of the output without the introduction of an additional
separate optic. The magnifying telescope could also employ mirrors
instead of lenses.

[0084]The foregoing discussion discloses and describes merely exemplary
embodiments. One skilled in the art will readily recognize from such
discussion, and from the accompanying drawings and claims, that various
changes, modifications and variations can be made therein without
departing from the spirit and scope of the invention as defined in the
following claims.